Consumers are demanding greater fuel economy and lower exhaust emissions from internal combustion engines. One way to increase fuel economy and reduce emissions is to improve the fuel system. A key component of the fuel system is the fuel injector, which injects pressurized fuel into the combustion chamber. Consequently, engine manufacturers are seeking improved fuel injectors for enhancing the performance of internal combustion engines.
To enhance engine performance, advance fuel injectors deliver fuel at higher injection pressures. They also provide better control of the injection rate and timing. To obtain the higher injection pressure, advanced fuel injectors operate with a higher hydraulic input pressure than earlier injector designs. The hydraulic input pressure can be as high as 6,000 psi. Advanced fuel injectors use the higher pressure to operate an intensifier piston for delivering fuel at injection pressures as high as 21,000 psi. To better control the injection rate and timing, advanced fuel injectors are electronically controlled and hydraulically actuated. Earlier injector designs are mechanically actuated.
Several advanced fuel injectors use a spool valve to control the flow of hydraulic fluid. FIGS. 1 and 2 show fuel injectors incorporating spool valves according to the prior art. Generally, spool valves are used in hydraulic and pneumatic systems to control and divert fluid flow. In a typical design, a spool valve has a spool positioned to reciprocate inside a valve body. To start fluid flow, the spool is moved to a position where the grooves on the spool are adjacent to grooves on the valve body. To stop fluid flow, the spool is moved to a position where lands on the spool are adjacent to lands on the valve body. The benefit of the spool valve is its ability to change the fluid flow with only a small change in the position of the spool. This ability to change the fluid flow provides better control and timing of the fuel injector.
To manufacture spool valves, pre-formed valve bodies and spools are machined individually according to specifications for the valve. Lathes, grinding machines, and other similar equipment cut grooves to form lands on the valve bodies and spools. The outside diameters of the spool are machine ground to match the inside diameters of the valve bodies. Finally, the valve body and spool are assembled. However, the precision with which the lands on the spool match the lands on the valve body is important for proper operation of the spool valve.
To compensate for mismatched lands, some applications adjust the position of the spool within the valve body to match the spool and body lands. This adjustment requires extra space for the spool in the valve body and usually a means to hold the spool in the proper position. Another approach is to increase the width of the lands to accommodate any mismatched lands. The larger width creates a larger "dead zone" in the spool valve. A larger dead zone reduces the transition time of the spool valve. This approach also increases the size of the spool valve. In some applications, mismatch lands have a minimal affect on the application. Sometimes, the control and timing offered by matched lands is of little benefit to the application.
In contrast, a spool valve for a fuel injector must have matched lands for proper timing and control. If the lands do not match properly, they may not stop or start hydraulic fluid at the appropriate time. They may start hydraulic fluid at an insufficient pressure or volume. They may cause to the hydraulic fluid to stop slower. Moreover, if the lands are not precisely matched in relation to the other lands, the spool valve may start or stop hydraulic fluid at one point before it stops or starts hydraulic fluid at another point. These problems affect the timing and control of the fuel injector as well as the energy efficiency of the engine. Improper diversions of hydraulic fluid from mismatched lands cause an energy loss in the engine. They also generate heat in the hydraulic fluid.
In addition, the higher hydraulic input pressure of advanced fuel injectors increases the adverse effects of mismatched lands. The higher hydraulic pressure aggravates any problem with the lands even if the condition only occurs in the brief microsecond when the spool is opened or closed.
Current methods to compensate for mismatched lands do not work well for spool valves used in fuel injectors. The size of the fuel injector does not permit lengthening the spool to expand the dead zones. In addition, there is little or no room to reposition the spool in the valve body for proper matching. A typical valve body has a length of 1.0 in. (25 mm). A typical spool has a length of 0.8 in. (20 mm) and a diameter of 0.25 in. (6.3 mm). Tolerances for successful operation of a typical spool valve are .+-.1.0.times.10.sup.-4 in. (0.0025 mm). These sizes and tolerances make regrinding difficult if not impossible.
Moreover, the "multiple" control operation of a spool valve for a fuel injector requires matching lands at multiple points almost simultaneously. In a typical fuel injector design, one set of lands must stop the flow of hydraulic fluid at the same time or slightly before a second set of lands starts the hydraulic flow. At another time, two or more sets of lands must start the hydraulic flow to hydraulic conduits at essentially the same time. In this design, simply repositioning or lengthening the spool will not correct for mismatched lands.
The need for matched lands makes the production of spool valves for fuel injectors very difficult. To obtain a properly working spool valve, the spool may be ground many times. Even then, the regrinding may not work because of the size and position of the lands on the spool.
Accordingly, there is a need for a method to match the lands in a spool valve for a fuel injector. In particular, there is a need to match the lands on the spool with the lands on the valve body when the spool valve is made.